1. Field of the Invention
This invention relates to a method of manufacturing a solid-state image pickup device for use in a photoelectric converter employing a photoconductive film, and more particularly to a method of manufacturing such a device having a low dark current and low lag characteristic.
2. Description of the Prior Art
A "two-level" solid-state image pickup device is a device having a signal charge reading-out portion constituted by a charge-coupled device (CCD) formed on a silicon monocrystal substrate, and a photoelectric conversion portion contituted by a photoconductive film formed on the signal charge reading-out portion. The "two-level" solid state image pick-up device has many advantages, such as increased sensitivity, due to its being able to present a large aperture to incident light, and low blooming. However, there are problems with the characteristics of such a construction as will be explained below.
FIG. 1 is a cross-sectional view of a conventional two-level solid-state image pickup device. Reference numeral 11 designates, for example, a p-type Si substrate, on the surface of which are provided a first n.sup.+ layer 12a providing the CCD channel for read-out of signal charge, a second n.sup.+ layer 12b constituting a signal charge accumulation region, and n.sup.++ layer 14 provided to establish an ohmic connection between this accumulation region 12b and a first metallic electrode 17a formed for example of aluminum (Al), and a channel stopper p.sup.+ layer 13. On top of a gate oxide film, which is itself provided on top of the CCD channel 12a, there are formed CCD transfer electrodes 15a, 15b, which are made, for example, of two layers of polycrystalline silicon. A first insulating film 16a is deposited on top of these two transfer electrode layers 15a and 15b. The first metallic electrode 17a, which is connected to the n.sup.++ layer 14 through contact holes formed in the insulating film 16a, is provided so as to extend over the CCD channel. A second insulating film 16b is then deposited on this, and second metallic electrodes 17b, formed of aluminum (Al), which are pixel electrodes, are provided for each pixel region. These second metallic electrodes 17b are connected to the first metallic electrodes 17a through contact holes 20 provided in the second insulating film 16b. The contact holes 20 are formed by a reactive ion etching (RIE) of the second insulating film 16b using a patterned photoresist as a mask. On top of the second metallic electrodes 17b, a photoconductive film 18 consisting of, for example, amorphous Si and a transparent conductive layer provided by an ITO film 19 are formed.
The n.sup.+ layer 12b constituting the signal charge accumulation region, n.sup.++ layer 14, first metallic electrode 17a and second metallic electrode 17b are separately provided for each pixel. That is, the second metallic electrodes 17b define image pickup pixel regions and the potential variations obtained by the second metallic electrodes 17b as a result of the image pickup performed by the photoelectric film 18 are transmitted through the first metallic electrode 17a to the n.sup.+ layer 12b constituting the accumulation region, and are accumulated in this region as signal charges. In the view shown in FIG. 1, the channel stopper p.sup.+ layer 13 can be seen, but transfer regions which transfer the signal charges of the n.sup.+ layer 12a are of course provided for each pixel.
In such a solid-state image pickup device, the substrate surface before the photoconductive film 18 is formed must be as flat as possible. If it is not flat, this results in appreciable dark current and lag of the solid-state image pickup device, with adverse effects on the image pickup characteristics.
The reason for this is that the film quality of the photoconductive film is poorer over substrate surface irregularities as compared with what it is where the substrate surface is smooth, and this increases the number of generation and recombination centers, which cause dark current and lag. It is therefore necessary to ensure that the surface of the second insulating film 16b is as flat as possible, and that the contact holds 20 formed in the second insulating film 16b provide as few irregularities as possible.
However, in the previous method, wherein the contact holes 20 were formed by RIE using a photoresist mask, the process was very complicated because many process steps such as forming a photoresist film, patterning thereof and etching of the insulating layer by RIE were necessary. Furthermore, in the previous method in which RIE was used, the sides of the contact holes were very steep so if the second metallic electrodes 17b were made thin, breaks in the metallic film became a considerable problem. Therefore, the minimum thickness of the metallic electrodes 17b was limited to approximately 4000 .ANG.-5000 .ANG., and irregularities of the substrate surface could not be minimized.